Explaining Geographic Range Size by Species Age 47age of species can help explain patterns of rarity
and endemism.
The potential importance of species age as
a predictor of range size was first championed
by Willis (1922). His “age-and-area hypothesis”
asserted that, on average, older species will have
larger ranges than younger species. He drew much
of his evidence from studies of the tropical flora of
Ceylon (now Sri Lanka) where he observed that
putatively ancestral species were more widely dis-
tributed than derived forms. Willis published a
number of papers on the subject, and his ideas
were subsequently debated and, in some cases,
even ridiculed (e.g., Fernald 1924, Gleason 1924).
In time, Willis’s hypothesis failed to gain support
(Stebbins and Major 1965) and his most lasting
influencemayactuallyhavebeeninphylogenetics,
via Yule’s (1925) seminal paper that mathemati-
cally derived a model of a pure-birth speciation
process, using Willis’s ideas as the theoretical
foundation.
Recently, the potential effect of historical
processes on the distribution and abundance
of organisms has received renewed attention
(e.g., Ricklefs 2004, Wiens and Donoghue 2004).
Much of this interest has been driven by two
factors: the influx of molecular data on organisms
that provide the potential to age the divergence
dates of species, and the publication of Hubbell’s
Neutral Theory of Biodiversity and Biogeography
(2001a), which incorporates the large-scale, long-
term effects of speciation and extinction on the
abundance and distribution of species. Hubbell’s
neutral theory also specifically predicts that most
rare, endemic species will be young species, while
most wide-ranging species will be old (Hubbell
2001a,b); in effect, Hubbell’s model makes a pre-
diction similar to Willis’s hypothesis. This predic-
tion can be viewed as a general expectation, rather
than a prediction specific to Hubbell’s model.
A positive relationship between species age and
range size can be expected if two assumptions are
met: (1) species start with small population and
range sizes; and (2) extinction risk is inversely pro-
portional to population and/or range size. Under
these assumptions, new (young) species will have
small population and range sizes and will face
a high probability of extinction, while species
that do persist and increase in range size will
face a decreasing probability of extinction. As a
result, on average, young species are expected to
be narrowly endemic species, while wide-ranging
species are expected to be old. Interestingly, some
of the strongest criticism of Hubbell’s neutral
model has focused on the expected age of com-
mon species. Specifically, if common species reach
high abundance via ecological drift, the expected
age of these species is unrealistically old, because
of the slow pace of drift (Leigh 1999, Ricklefs
2003, Nee 2005). In contrast, if fitness devia-
tions are accepted in the model, species can reach
high abundance or go extinct much more quickly
(e.g., Yuet al. 1998, Fuentes 2004). As a result,
a positive age and range size relationship may be
expected to persist much longer in clades that have
been primarily driven by neutral processes than in
cladeswhereselectionhasdrivenspecieswithhigh
relative fitness to occupy large ranges.
Of course, the relationshi pbetween s pecies age
and range size may take many forms, and Willis’s
age-and-area hypothesis (1922) is only one of
several models of post-speciation range-size trans-
formation. For example, Gaston and colleagues
(Chown 1997, Gaston 1998, 2003, Chown and
Gaston 2000) have summarized a series of mod-
els of post-speciation range-size transformations
(e.g., cyclical, random, stasis, etc.) that could
potentially better explain the age and area rela-
tionshipsof somespecies.Forexample,theageand
area relationship may be explained by a hump-
shaped curve, where species start with small range
sizes, reach their maximum range size at an inter-
mediate age, and then decline towards extinction
when they are old. Such a pattern was found for
the proportion of fossil assemblages occupied by
Cenozoic mollusks (Footeet al. 2007). Because
there are a variety of processes that can expand or
reduce species’ ranges, individual clades may have
their own unique age–area relationships. Thus,
the utility of species age as a broad explanatory
variable remains to be seen. In this chapter, we
briefly review the few empirical tests of age and
area and present an analysis using a clade of trop-
ical understory shrubs (Piper). We discuss how
the species age and range size relationship can be
viewed more broadly than the simple hypothe-
sis presented by Willis (1922) and how this can
lead to new hypotheses and understandings of